This invention relates to telephone systems and, more particularly, to range extenders for improving signaling and transmission on communications channels.
Numerous communications channels, for example, telephone transmission lines, consist of a twisted pair of leads. Signaling and transmission range of such transmission lines is limited by attenuation caused by the line resistance. The longer the line the greater the attenuation and, hence, the greater the distortion of telephone signaling and transmission.
Basically, signaling is the transfer of nonvoice information that controls processing of a telephone call, for example, signaling includes supervision, dial pulsing, ringing and ring-trip. The basic signaling problem in long transmission lines is that there may not be sufficient line current to assure operation of central office detection circuits, for example, relays and the like. Similarly, the basic transmission problem is loss of transmitter efficiency because of low "talk" battery current, i.e., low loop current. Various solutions have been proposed to overcome these difficulties.
One particularly advantageous solution to the line signaling and transmission problems is the polarity sensitive boost voltage insertion circuit disclosed in U.S. Pat. No. RE 27,680, issued to I. M. McNair, Jr., on June 19, 1973. Basically, the polarity of a central office battery potential applied to the leads of a transmission line is detected and a potential is inserted in the line to series aid the central office battery. Equal amplitude potentials are inserted in both leads of the line in an attempt to maintain longitudinal balance of the line impedance. Both voltage polarity sensing and current flow detection arrangements are disclosed. The voltage polarity sensing arrangement typically requires a shunt resistance connected between the leads of the line as a voltage sensing element and a fairly complex switching arrangement employing a plurality of transistors. Use of a shunt resistor is somewhat undesirable because of the loss of isolation between the tip and ring leads of the transmission line. In the current sensing arrangement, line current is employed to develop bias potentials to drive transistors for inserting appropriate boost potentials. Complementary transistor circuit arrangements are used to insert the potentials in both sides of the line.
Another problem with both the prior known voltage sensing and current sensing arrangements is so-called longitudinal imbalance of the transmission line. It is well known that long transmission lines develop 60 Hz. hum signals on the individual leads. If the impedance of the leads is in balance, the hum signals will eventually be cancelled. Therefore, it is important that the impedance inserted into both leads of the line, when inserting the boost potentials, is substantially the same. If not, an imbalance results in the amplitude of the undesirable hum signals developed on the line. This imbalance in longitudinal hum signals is eventually converted into a so-called metallic current signal which degrades transmission quality over the line. The use of complementary transistors, i.e., NPNs in one lead and PNPs in the other lead, in the prior known arrangements to insert boost potentials into the line tends to cause undesirable longitudinal imbalance on the line.
Still another problem with prior known boost potential insertion arrangements relates to the insertion of potentials during the ringing cycle. It is important that the potential inserted in the transmission line series aid the central office battery during the ringing cycle. In practice, ringing is usually effected by applying a negative DC potential to the ring lead of a telephone transmission line and by superimposing a AC ringing signal thereon. Because of the relative amplitudes of the DC and AC potentials, the combined potential applied to the ring lead becomes positive during portions of the ringing interval. This positive transition appears to the range extender as an apparent reversal in polarity of the central office battery potential. Consequently, prior known insertion arrangements would reverse the inserted potential or potentials. Any such reversal of the inserted potential or potentials during ringing is undesirable because it would effectively buck the central office battery potential rather than series aid it and, therefore, interfere with the ring trip function of central office and/or subscriber equipment.